1. Field of the Invention
The present invention relates generally to guidance and control systems, and more particularly, to methods and devices for providing guidance and control of low and high-spin rounds.
2. Prior Art
Since the introduction of 155 mm guided artillery projectiles in the 1980's, numerous methods and devices have been developed or are under development for guidance and control of subsonic and supersonic rounds. These include different technologies and related components such as actuation devices, position and angular orientation sensors, and guidance and control hardware and algorithms. The majority of these devices have been developed based on missile and aircraft technologies, which are in many cases difficult or impractical to implement on gun-fired projectiles and mortars. This is particularly true in the case of actuation devices, where electric motors of various types, including various electric motor designs with or without gearing, voice coil motors or solenoid type actuation devices used to actuate control surfaces have dominated the guidance and control of most guided weaponry. Thrusters of various types have also been successfully employed. However, currently available thrusters are suitable only for low or no-spin rounds due to their limitations in terms of relatively long pulse widths and unpredictable actuation delays. Other currently available actuation technologies developed for munitions applications are suitable for non-spinning rounds or for rounds with very low spinning rates.
Current guidance and control technologies and those under development are not effective for flight trajectory correction of high-spin guided munitions. Such spin stabilized rounds may have spinning rates of 200 Hz or higher, which pose numerous challenging sensing, actuation and control force generation and control algorithm and processing issues that need to be effectively addressed using innovative approaches. In addition, unlike missiles, all gun-fired spinning rounds are provided with initial kinetic energy through the pressurized gasses inside the barrel and are provided with flight stability through spinning and/or fins. As a result, they do not require in-flight control action for stability and if not provided with trajectory altering control actions, such as those provided with control surfaces or thrusters, they would simply follow a ballistic trajectory. This is still true if other means such as electromagnetic forces are used to accelerate the projectile during the launch or if the projectile is equipped with range extending rockets. As a result, unlike missiles, control inputs for guidance and control is required only later during the flight and in many cases as the projectile approaches the target.
In recent years, alternative methods of actuation for flight trajectory correction have been explored, some using smart (active) materials such as piezoelectric ceramics, active polymers, electrostrictive materials, magnetostrictive materials or shape memory alloys, and others using various devices developed based on micro-electro-mechanical (MEMS) and fluidics technologies. In general, the available smart (active) materials such as piezoelectric ceramics, electrostrictive materials and magnetostrictive materials (including various inch-worm designs and ultrasound type motors) need to increase their strain capability by at least an order of magnitude to become potential candidates for actuator applications for guidance and control, particularly for gun-fired munitions and mortars. In addition, even if the strain rate problems of currently available active materials are solved, their application to gun-fired projectiles and mortars will be very limited due to their very high electrical energy requirements and the volume of the required electrical and electronics gear. Shape memory alloys have good strain characteristics but their dynamic response characteristics (bandwidth) and constitutive behaviour need significant improvement before becoming a viable candidate for actuation devices in general and for munitions in particular, even those with very low spin rates.
All currently available actuation devices based on electrical motors of various types, including electrical motors, voice coil motors and solenoids, with or without different gearing or other mechanical mechanisms that are used to amplify motion or force (torque), and the aforementioned recently developed novel methods and devices (based on active materials, such as piezoelectric elements, including various inch-worm type and ultrasound type motors), or those known to be under development for guidance and control of airborne vehicles such as missiles, suffer from the basic shortcoming of not being capable of providing the dynamic response levels that are required for guidance and control of high-spin rounds with spin rates of up to 200 Hz or higher. This fact is readily illustrated by noting that, for example, a round spinning at 200 Hz would undergo 72 degrees of rotation in only 1 msec. This means that if the pulse duration is even 1 msec and its unpredictable initiation time (pulse starting time) is off by 1 msec, then the direction of the effective impulse acting on the round could be off by over 90 degrees, i.e., when a command is given to divert the round to the right, the round may instead be diverted up or down. Such a level of uncertainty in the “plant” (round) trajectory correction response makes even the smartest feedback control system totally ineffective.
The most important sensory input for a guidance and control system of a high-spin round is that of the roll angle measuring sensor. Roll angle measurement in munitions has been a challenge to guided munitions designers in general and for high-spin rounds in particular. The currently available laser gyros are impractical for use in munitions due to size, cost and survivability. Magnetometers are also impractical since they can only measure angle in two independent directions, which may not be aligned for roll angle measurement at all times during the flight. Their angle measurement is also not precise and requires a local map and is susceptible to environment in the field. Inertial based gyros may be used, but require initiation at regular time intervals to overcome initial settling and drift issues.
In summary, the currently available guidance and control systems and their components suffer from one or more of the following major shortcomings that make them impractical for application to high-spin guided munitions:
1. Limited Dynamic Response: The munitions with high spin rates demand control actuation of any type to provide very short duration (sub-millisecond) “pulses” in order for the control action to be applied over only a limited range of munitions roll angle. For a round spinning at 200 Hz, if the control actuation is to be applied over a 10 degrees range of roll angle, then the control actuation must be applied for only around 0.14 milliseconds, or at an equivalent frequency of around 7,200 Hz. This would obviously eliminate any of the aforementioned currently available actuation devices for such high-spin round guidance and control applications.
2. Actuation Pulse Timing and Duration: In addition to the above dynamic response limitations, the fastest thruster or impulse type guidance and control actuation devices that are currently available suffer from two basic shortcomings: (1) actuation pulse timing precision; and (2) pulse width precision. The first shortcoming is mainly due to unpredictable delays in the initiation devices, while the second shortcoming is mainly due to the relatively long pulse durations in commonly used thrusters or the like in current technologies.
3. Roll Angle Measurement: An effective guidance and control technology for high-spin rounds requires sensors for onboard measurement of the projectile roll angle. The roll angle sensor has to provide the require precision and should not be subject to drift or other similar effects that over time during the flight causes error to accumulate and render roll angle measurement unreliable. It is also appreciated that one may use roll angle sensors that are subject to drift and exhibit relatively long settling times, but in such cases, appropriate means have to be provided for initialization of the sensor at regular and often time intervals.
4. High Power Requirement: All currently used actuation mechanism working with electrical motors and/or solenoids of different types as well as actuators based on active materials, such as piezoelectric materials and electrostrictive materials and magnetostrictive materials (including various inch-worm designs and ultrasound type motors) and shape memory based actuator designs, are only applicable to munitions with low spin rates. But even in such applications, they demand high electrical power for their operation.
5. Occupy Large Munitions Volume: One solution that has been employed or has been considered for high-spin guidance and control has been de-spinning the entire round or a section of the round where the control surface or the like are positioned. As a result, the aforementioned issues with high-spin rates are resolved. Such solutions are, however, impractical for medium caliber munitions due to the lack space to provide the means to de-spin the round. Such solutions are practical for larger caliber rounds, but even for these cases they are highly undesirable for the following reasons. Firstly, the actuation devices and mechanisms required for de-spinning occupy a significant portion of the round volume. The available volume for payload is also further reduced since fins (or larger fins) or other stabilizing means must also be provided to ensure stable flight. As a result, the weapon lethality is significantly reduced. In addition, a significant amount of power has to be provided for de-spinning of the round.
6. High cost of the existing technologies, which results in very high-cost rounds, thereby making them impractical for large-scale fielding.
7. Relative technical complexity for the implementation of the current guidance and control technologies for high-spin rounds such as for de-spinning of the entire round or its guidance and control section, which results in increased munitions cost.